I've just run into a problem with my Flex Debugger - it basically stopped working. I tried reinstalling the Flash Debug Player and also tried restarting Flex Builder with the -clean option. All to no avail.
A Quick Google search however brought up this and this. It seems that several Firefox add-ons can break the Flex Debugger (this seems to be a bug with Firefox 3's handling of add-ons). For me personally, disabling the Google Toolbar add-on fixed the Flex Debugger. Other users reported that disabling Greasemonkey or Adblock provided a fix. I can live without the Google Toobar, but I cannot live without the Flex Debugger...

By the way, here's how to start Flex Builder with the -clean option, it can sometimes solve issues - though not in this case.
Run Terminal (/Applications/Utilities/Terminal)
Then enter:

view plain print about
1cd "/Applications/Adobe Flex Builder 3/Flex Builder.app/Contents/MacOS"
2
3./FlexBuilder -clean